Interplay of Segmental and Normal Mode Dynamics in Polymer Networks Undergoing Chemical Cross-Linking. Epoxy/Amine-Terminated Linear and Star PPO Formulations

An investigation was conducted of segmental and normal mode dynamics during cross-linking of reactive systems where one of the components exhibits, in addition to the transverse dipole moment (μ ⊥ ) component that gives rise to the segmental α process, a persistent cumulative dipole moment (μ ∥ ) along the chain contour that can be relaxed via the normal mode process. The systems studied were composed of an amine-terminated linear or three-arm star poly(propylene oxide), which contains both μ ⊥ and μ ∥, and a bifunctional epoxy prepolymer. The kinetics of network formation were evaluated by Fourier transform near-infrared spectroscopy (NIR), and the dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS). The dynamics of networks containing linear and star chains were similar but not identical. The average relaxation time for segmental (τ S ) and normal mode (τ N ) increases in the course of network formation, but the distance between τ S and τ N varies little and the T g -scaled fragility remains unchanged. The spectra become thermodielectrically complex following the onset of reactions and broaden in the course of cure. Segmental and normal mode relaxations overlap increasingly during cure but, interestingly, retain their identities. There is a decrease in the dielectric relaxation strength for the segmental process (Δε S ) and a simultaneous (unexpected) increase in the dielectric relaxation strength for the normal mode process (Δε N ). Before gelation, the DMS response was characterized by segmental and terminal relaxation zone. The gel point was observed at a conversion above that predicted by the gelation theory, and an explanation was put forward.